This invention relates to improved light alloy articles having a heat resisting and insulating surface layer and adapted for use as automobile parts such as internal combustion engine pistons and combustion chamber-defining cylinder heads; and a method for manufacturing the same.
As is well known in the art, the so-called light alloys such as aluminum alloys and magnesium alloys are characterized by their light weight and good heat conduction, and have been widely used in the manufacture of members and parts which need such properties. These light alloys, however, are undesirable for the manufacture of those parts which are subject to elevated temperatures because the light alloys themselves have a low melting temperature and poor heat resistance. These light alloys are also unsuitable for the manufacture of those parts which are required to be heat insulating because their heat conduction suggests, on the other hand, that they are poor heat insulators. To eliminate these shortcomings in order that light alloys may be used in the manufacture of those parts which require heat resistance and insulation as well as light weight, for example, internal combustion engine pistons and combustion chamber-defining cylinder heads, attempts have heretofore been made to provide a light alloy body with a heat resisting and insulating layer on its surface. For the manufacture of internal combustion engine pistons, for example, a light-weight aluminum or magnesium alloy is used as a base for the piston and a coating material having high heat resistance as well as low heat conductivity, such as ceramic and refractory metal is applied to a head portion of the piston, thereby preventing the melting- or burning-away of the head portion as well as reducing thermal loads to the piston and associated piston rings and cylinder. Such heat resisting and insulating piston heads recently become of more interest from a standpoint of improving combustion efficiency or the like.
The previously proposed methods for applying a heat-resisting and -insulating surface layer to a head portion of a piston body made of light alloy such as aluminum and magnesium alloys are generally classified into the following three types. The first method is by preforming a ceramic material or a refractory metal such as a Nb base alloy, W base alloy and Mo base alloy, and joining the preform to a piston body of light alloy by mechanical fastening (e.g., bolt fastening and crimping) or welding. The second method uses insert casting process by which a ceramic material or refractory metal is integrated with a piston body of light alloy. The third method is based on surface coating techniques including metallization or spraying, anodization and electrodeposition. A head portion of a light alloy piston body may be coated with a ceramic material or refractory metal by any of these techniques.
In providing the piston head portion with a surface layer for heat resistance and insulation, important are the following factors: (1) light weight, or no sacrifice of the light weight of the piston body, (2) high heat resistance and insulation, (3) high durability, or prevention of the surface layer from cracking or peeling from the piston body, (4) ease of manufacture, and (5) low cost. However, none of the above-mentioned conventional methods have succeeded in fully satisfying these requirements. More specifically, in the first or second method, a refractory metal having a coefficient of thermal expansion approximating to that of the light alloy of which the piston body is made may be selected and it can be joined to the light alloy more firmly than ceramic materials are, leading to an advantage in durability. However, since the refractory metal is poorer in heat insulation and fire resistance than ceramic material, the refractory metal layer must be increased in thickness. The increased thickness of the refractory metal layer along with the considerably higher specific gravity of refractory metal itself than the bulk specific gravity of ceramic material results in an undesirable increase in weight of the piston. On the other hand, when ceramic materials are used in the first or second method, some advantages are obtained including light weight, heat insulation and fire resistance. However, because of their coefficient of thermal expansion significantly different from those of light alloys such as aluminum and magnesium alloys, the ceramic materials are susceptible to cracking or failure during service. The use of ceramic materials thus encounters some difficulty in forming a durable ceramic cover. Durability may be improved only at the sacrifice of cost. Furthermore, finishing of the ceramic material to a predetermined shape further increases the cost because of its poor processability.
The third method, that is, surface coating method also suffers from serious problems. Coatings resulting from anodization or electrodeposition can be at most 0.1 mm in thickness, which is too thin to provide sufficient heat insulation and fire resistance. The spraying or metallizing involved in the third method allows coatings to be increased in thickness in comparison with the other surface coating techniques, for example, up to as thick as 2 mm. Thicknesses of such an order are still insufficient to achieve practically acceptable heat insulation and resistance when metallic materials are used. Ceramic base materials should be selected for this reason. Because of its difference in coefficient of thermal expansion from the light alloy of which the piston body is made, the ceramic coating is susceptible to cracking and peeling during service as in the above-mentioned case, leaving a durability problem. As a countermeasure, it is known to spray a certain metal to the surface of a light alloy piston body to form an intermediate layer, the metal having high heat resistance and a coefficient of thermal expansion intermediate that of the light alloy and a ceramic material to be subsequently sprayed, for example, Ni-Cr alloy, Ni-Cr-Al alloy, and Ni-Cr-Al-Y alloy. A ceramic material is then sprayed onto the intermediate layer such that the intermediate layer may compensate for a difference in thermal expansion between the overlying ceramic layer and the underlying light alloy piston body. Since the intermediate layer generally has a thickness of 100 .mu.m or less, it is insufficient to absorb the thermal expansion and contraction of the piston body. There still remains unsolved a durability problem.
Therefore, an object of the present invention is to provide improved light alloy articles which take advantage of the inherent light weight of light alloys themselves, have excellent heat resistance, heat insulation and durability, and can be produced less costly in high yields. Another object of the present invention is to provide a method for producing such improved light alloy articles.